1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus having the zoom lens. More specifically, the present invention relates to a zoom lens useful as a photographic (shooting) optical system of an image pickup apparatus, such as a digital camera, a video camera, a television camera, and a silver-halide film camera.
2. Description of the Related Art
In recent years, it is desired by the market that an image pickup apparatus that uses a solid-state image sensor, such as a video camera, a digital still camera, or a silver-halide film camera, has a large number of functions. Furthermore, it is desired by the market that the size of the entire apparatus is small at the same time and that a photographic optical system (zoom lens) used in such an image pickup apparatus has a wide angle of view, is small in total size, and the total weight thereof is light.
In an image pickup apparatus using a solid-state image sensor, various optical members, such as a low-pass filter and a color correction filter, are provided between a rearmost portion of the lens and the image sensor. In addition, in an image pickup apparatus, such as a single-lens reflex camera, a quick return mirror is provided between the last (rearmost) lens surface and the image sensor. Accordingly, it is desired that a photographic optical system used in the image pickup apparatus described above has a long back focus.
A conventional negative-lead type zoom lens, which is a zoom lens having a wide angle of view and a long back focus and whose entire size is small, includes a lens unit having a negative refractive power at a location closest to the object side. U.S. Pat. No. 6,081,389 and Japanese Patent Application Laid-Open No. 06-273670 each discuss a negative-lead type zoom lens composed of two lens units having, in order from the object side to the image side, negative and positive refractive powers.
Japanese Patent Application Laid-Open No. 2000-330024 and Japanese Patent Application Laid-Open No. 2003-177314 each discuss a negative-lead type zoom lens composed of three lens units having, in order from the object side to the image side, a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, and a third lens unit having a negative refractive power.
In addition, Japanese Patent Application Laid-Open No. 2006-58584 and Japanese Patent Application Laid-Open No. 2001-42217 each discuss a negative-lead type zoom lens composed of four lens units having, in order from the object side to the image side, negative, positive, negative, and positive refractive powers.
The first lens unit of a negative-lead type zoom lens, may have a large effective diameter and a relatively heavy weight. In order to reduce the size and weight of the first lens unit, Japanese Patent Application Laid-Open No. 2002-072091, U.S. Pat. Nos. 6,992,835, 6,229,655, and Japanese Patent Application Laid-Open No. 2007-156043 each discuss a zoom lens including a small-size and lightweight first lens unit having negative, negative, and positive lenses appropriately made of a plastic lens material.
In particular, an aspheric lens made of a plastic material can be relatively easily manufactured and is relatively light in weight. Accordingly, if an aspheric lens made of a plastic material is used as a lens of the first lens unit, the size and weight of the first lens unit can be easily reduced.
However, if the refractive power arrangement and the lens shape are not appropriately set when an aspheric lens made of a plastic material is used as a lens of the first lens unit, it becomes difficult to achieve a high optical performance for the entire zooming range, reduce the size of the entire zoom lens system, and provide a wide angle of view.